This invention relates to a method for determining the amount and character of rotary drilling bit wear in situ and without interruption of the drilling operation. Although not limited to a specific type of drilling bit, this method has proved quite useful in evaluating the wear of rotary cone-type drilling bits.
One of the most common drilling bits for use in penetrating earth formations such as oil and natural gas wells is the roller-cone rock bit which comprises a plurality of toothed cutting cones which are rotatably mounted to a corresponding number of depending support arms commonly called "shirt tails". As the entire assembly is rotated, the cutting cones or wheels roll on the bottom of the borehole thereby chipping away the rock formations. As the bit advances downward, the loosened rocks and dust is carried away by the drilling fluid.
Bit wear in rotary drilling operations is a considerable problem for a number of reasons, perhaps the primary one being the fact that due to the large daily operation expenses, an optimum drilling rate is of the utmost importance. Since a worn bit will result in a slower drilling rate, a point is reached where it is economically desirable to pull the string and change the bit. Since this operation is quite time consuming and causes interruption of the drilling, the decision to change the bit must be made prior to pulling the drill string and repeated inspections of the drill bit are not possible. Also, a worn bit might be mistaken for an abnormally hard formation when in fact the rate might be increased by simply changing the weight and rotary speed of the bit.
An excessively worn bit may also result in the loss of a cone, thereby necessitating an expensive and time consuming fishing operation for its recovery. Also, a worn bit will generally cause the borehole to be under-gauge which requires extreme caution in lowering the new bit to the bottom of the borehole because the under-gauge condition will cause pinching and rapid deterioration of the new bit unless the hole is first properly reamed.
SINCE IT IS HIGHLY IMPRACTICAL TO PULL THE DRILL STRING IN ORDER TO PERIODICALLY CHECK THE BIT, A NUMBER OF METHODS IN THE PAST HAVE BEEN PROPOSED FOR DETERMINING THE DEGREE OF BIT WEAR IN SITU. One of the prior art methods entails the release of a dye or other detectable substance from the bit when a certain degree of wear has been achieved. This substance is released into the drilling fluid and carried to the surface where it is detected either by physical inspection or by an automatic detector. Other prior art solutions to the problem are the measurement of the loading, torque, or speed of the rotating bit and cones and then correlating this information to expected values of known conditions of bit wear. A difficulty with these last mentioned methods is that the bit wear is detected by measuring the performance of the bit itself or in some cases the entire string. This lends itself to misinterpretation because the performance of the drilling rig will often be affected by other factors not related to bit wear. Another difficulty with many of the prior art methods is that they indicate only the degree of wear and not the nature of the wear nor the most likely cause. This last aspect is quite important when abnormal bit wear occurs because it provides the opportunity to correct the situation prior to extensive damage to the bit.